Embed Code

Computer Networks PowerPoint PPT Presentation

Computer Networks. Chapter 6 - Multiplexing. Multiplexing. The term “multiplexing” is used whenever it is necessary to share a single device among several devices

Copyright Complaint Adult Content Flag as Inappropriate

Download Presentation

Computer Networks

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

Presentation Transcript

Multiplexing

The term “multiplexing” is used whenever it is necessary to share a single device among several devices

In this chapter multiplexing denotes a set of techniques that provide sharing of a single high bandwidth channel among a number of individual users.

The reasons for using multiplexing

Media with high bandwidth are expensive

Sharing is necessary to justify their use

Computer Networks

Categories of Channel Multiplexing

FDM (Frequency Division Multiplex)

The individual user signals are shifted to different frequency channels

WDM (Wavelength Division Multiplex)

Conceptually the same as FDM, except that the multiplexing involves optical signals transmitted over fiber-optic channels

TDM (Time Division Multiplex)

Each user signal is assigned a particular time slot for transmission

Computer Networks

Other Types of Multiplexing

Statistical multiplexing

The channel is shared in time but without particular time slots assigned. Sharing is on the “as needed” basis.

DMT (Discrete Multitone)

Found in digital subscriber lines. It combines hundreds of different signals that are QAM modulated into a single stream

CDM (Code Division Multiplex)

An advanced technique that allows multiple devices to transmit on the same frequency at the same time

Computer Networks

FDM Multiplexing Process

Time domain

Frequency domain

Computer Networks

FDM –Demultiplexing Process

Time domain

Frequency domain

Computer Networks

The Analog Hierarchy

Computer Networks

Using FDM

The oldest multiplexing technique. Susceptible to noise

Used in the old days of telephone network to make efficient use of the long distance trunks

Used in AM and FM radio broadcast to provide the use of many stations on a single channel without each interfering with the other

The first generation (1G) of mobile phones used FDM with analog frequency modulation which required 2x30Khz per user

Computer Networks

Example:

FDM multiplexing is used for voice channels (each requiring 4000 Hz) with 200-Hz guard band between each signal source. If the total bandwidth is 29.6 KHz, how many channels can be multiplexed?

Answer: If n is the number of channels that can be multiplexed then the total bandwidth required i expressed with the following equation.

nx4000 + (n-1)x200 = 29600

Solving this equation for n will show that 7 channels can be multiplexed.

n= (29600-200)/(4000+200) = 29400/4200=7

Computer Networks

TDM – Time Division Multiplex

Digital process that is nowadays used for long-distance telephone lines.

A portion of time is allocated to each voice channel

The voice signal is first transformed into bits using PCM

The data flow is divided into units called frames. The frame contains as many bits as the number of channels multiplexed

Computer Networks

TDM – The Multiplexing Process

The duration of the bit on the high speed channel is 3 times shorter, assuming that three channels are multiplexed.

Computer Networks

TDM -Interleaving

Even when a channel has nothing to send, its time slot cannot be used by another channel

Synchronization between sender and receiver is important

Computer Networks

Example

20 voice signals are to be multiplexed and transmitted over twisted pair.

What is the bandwidth required, in bps, if synchronous time division multiplexing is used, if we use the standard analogue-to-digital sampling rate, if each voice signal has a bandwidth of 4000Hz, and if each sample is converted into an 8-bit value?

Answer: Each voice signal is sampled at 2 times the frequency = 8000 samples per second.

Each sample is an 8 bit value so:

8000 samples per sec × 8 bits = 64,000bps

20 signals × 64,000bps = 1,280,000bps.

Computer Networks

TDM - Flexibility

It is possible to assign several slots to a fast transmitting device

The channel needs to have the speed that is an integer times higher than the speeds of the regular channel

If this cannot be accomplished, extra dummy bits are added. This process is called bit padding

Computer Networks

Digital Hierarchy

Telephone companies group TDM slots in higher rate services that are standardized.

Computer Networks

American vs. European Standards

European

American

Computer Networks

Statistical Multiplexing

Statistical multiplexing is mainly used with computer networks

Only the data from active sources are transmitted using the full capacity of the channel

Bits are grouped into frames, but they can be with different size.

If there is more traffic than the high speed link can transmit, the frames are queued and are served using the first-come firs-served principle.

Computer Networks

Statistical Multiplex -Buffering the Packets

Packets for one output

Bursty flows share the same link efficiently

When too many packets arrive simultaneously, the buffer could not have enough space to keep all of them